The present invention relates to an optical disk apparatus for optically reproducing information from an optical recording medium, and more specifically, to an optical head for reproducing signals from optical disks whose substrate thicknesses are different from each other, and also to an objective lens employed therein.
Very recently, optical disks have greatly advanced in use as large memory changeable information recording media. Accordingly, there are many different sorts of recording/reproducing systems using disks of differing recording densities and disk sizes. Compatibility among them can hardly be secured. In particular, CD-ROMs (Compact Disk-Read Only Memorys) have been largely utilized. Thus, a strong demand for compatibility with a CD-ROM is required when optical disks are newly developed. As a next generation ROM with high density subsequent to this CD-ROM, a DVD-ROM (Digital Video Disk Read Only Memory) has been proposed very recently.
To increase the recording density in a DVD-ROM, a numerical aperture (NA) of an objective lens is increased from 0.45 (namely, for a conventional CD) to 0.6. Assuming now that a wavelength of laser light in use is selected to be ".lambda.", a dimension of a focused spot on an optical disk is directly proportional to .lambda./NA. To this end, as the NA is increased, the light spot can be made small accordingly. Assuming that the light spot is small, information pits of high density can be reproduced with better quality, so that the recording density of the optical disk can be enhanced.
However, as the NA is made large, the coma aberration that occurs when the disk is inclined is rapidly increased, so that the collective spot on the optical disk is conversely deteriorated. Therefore, the NA cannot be made excessively large. As a consequence, in the DVD-ROM, such a proposal has been made that the substrate thickness of the disk is made thinner than that of the CD-ROM, namely from 1.2 mm to 0.6 mm. Since the coma aberration that occurs when the disk is inclined is directly proportional to the thickness of the disk substrate, a too thin substrate may cancel the increase in coma aberration caused when the NA is increased.
To the contrary, when the thickness of the substrate of the DVD-ROM is made different from that of the CD-ROM, the compatibility between the DVD-ROM and the CD-ROM can be hardly maintained. This is because when the light is collected through the substrate of the optical disk, spherical aberration may occur under this condition. Accordingly, the objective lens for the optical disk is previously designed in such a manner that the objective lens compensates for the spherical aberration in accordance with a specific substrate thickness. When, however, information on a CD having a substrate thickness of 1.2 mm is reproduced by employing an objective lens optimized based on a substrate thickness of 0.6 mm, for example, spherical aberration will occur due to a thickness shift in the substrate of 0.6 mm, so that the light spot may have blooming, and thus no signal can be reproduced.
The conventional means for solving this problem is described in, for instance, OPTICAL REVIEW vol. 1, No. 1 (1994), pages 27-29, and Mitsubishi Electric Company News Release, Kaihatsu No. 9507 (Jun. 21, 1995).
The former conventional means is realized with a hologram that is formed on the surface of the objective lens for 0.6 mm, whereby the information of a CD is reproduced by the diffraction light of this hologram, and the DVD is reproduced by the transmission light. In this conventional means, the pattern of the hologram is previously designed to compensate for the spherical aberration that occurs when the information of a CD is reproduced.
In the latter conventional means, both the objective lens for 0.6 mm and the objective lens for 1.2 mm are mounted on the optical head. The two objective lenses are switched by the movable actuator to use the selected lens.
The above-described conventional apparatus has the following problems.
In the former means, since the hologram is used, even when the CD is reproduced, the light spot for the DVD is produced, whereas even when the DVD is reproduced, the light spot for the CD is produced. Also, the light reflected from the disk is diffracted on the disk. These operations may cause the light amount to be lost. In particular, this may cause a great problem when a rewritable type DVD is developed in the future.
In the latter case, since the two lenses are switched, the employment of such two lenses may produce various problems, i.e., high cost, deterioration in the positional reproducibility of these lenses, and also deterioration in response characteristic caused by employing a heavy and large actuator.
In light of these problems, an object of the present invention is to precisely reproduce a signal of a CD having a substrate thickness of 1.2 mm, and also a signal of a DVD having a substrate thickness of 0.6 mm at a low cost without any loss in light amount.
To solve the above-described problems, in an optical head comprising: a light source; an objective lens for focusing light from the light source to an optical information recording medium; a light branching element for branching reflection light reflected from an optical recording medium from a light path returned to the light source; a photodetector; and means for positioning a focused spot onto an information pit array of the optical recording medium; root mean square wave front aberration Wrms, a wavelength .lambda..sub.2 of the light source, and a numerical aperture NA.sub.2 of the objective lens may satisfy; ##EQU1## where characters "n.sub.1 " and "n.sub.2 " shows refraction indices of information recording medium substrates with respect to wavelengths .lambda..sub.1 and .lambda..sub.2 ; symbol "d" denotes a depth of an information pit; a light source wavelength of an optical head for recording or reproducing information from the optical information recording medium under such a degree that aberration can be optically neglected; and a numerical aperture of the objective lens is NA.sub.1.
Also, in an objective lens for focusing laser light onto an information recording film surface for recording, or reproducing information via a transparent parallel plate substrate on, or from an optical information recording medium for optically recording/reproducing the information, a thickness of said substrate for focusing the laser light and independently having aberration of the same conditions is different at a central portion and a peripheral portion.
Also, in an objective lens for focusing laser light having a wavelength of .lambda..sub.2 on the information recording film surface in order to record, or reproduce the information from two sorts of optical information recording mediums having different substrate thicknesses, different information pit hole depths, and different recording density; a peripheral portion develops no aberration when the laser light is focused through a substrate thickness of a first optical information recording medium with large recording density; a central portion develops no aberration when the laser light is focused through a substrate thickness between two sorts of substrate thicknesses; a focused spot root mean square wave front aberration of incident light in combination with light passing through the central portion is lower than, or equal to approximately 0.04.lambda. (".lambda." is a wavelength of laser light in use) with respect to the substrate of the first optical information recording medium; and while recording, or reproducing a second optical information recording medium having low recording density by way of light passing only through the central portion, a numerical aperture NA.sub.2 only about the central portion, a substrate thickness by which the light of the center portion is collected without any aberration, and also wave front aberration W.sub.rms caused by an error in the substrate thickness of the second optical information recording medium may substantially satisfy the above-described formula 1 (note that "n.sub.1 " and "n.sub.2 " indicate refraction indices of information recording substrates with respect to wavelengths .lambda..sub.1 and .lambda..sub.2) in comparison with an optical head having a light source wavelength .lambda..sub.1 under which information is optically recorded, or reproduced on, or from the second optical information recording medium without any aberration under better conditions, and a normal objective lens of a numerical aperture NA.sub.1.
Actually, in the case that the substrate thickness of the second optical information recording medium is approximately 1.2 mm, the objective lens may substantially satisfy the following expression: ##EQU2## In these objective lenses, the Abbe's sine condition as an overall objective lens, may be substantially satisfied.
Also, in these objective lenses, a shape of a boundary portion between the peripheral portion and the central portion is smoothly connected.
In these objective lenses, an optimum substrate thickness of the central portion is continuously variable in a coaxial shape from a lens center.
Also, in these objective lenses, a thin film is loaded on either the peripheral portion or the central portion, so that the optimum substrate thickness of the central portion is different from that of the peripheral portion.
Also, these object lenses are arranged by a lens whose substrate thickness is optimized over an entire surface, and a parallel plate arranged on the side of an object, a hole being formed in a central portion of the parallel plate.
An optical head capable of reproducing a signal from an optical recording medium having a substrate thickness of 1.2 mm and also a signal from an optical recording medium having a substrate thickness different from 1.2 mm is constituted by employing these objective lenses.
Also, signals can be reproduced from the optical disk having the substrate thickness of 1.2 mm and from the optical disk having the substrate thickness different from 1.2 mm without replacing the objective lens. While the optical disk having the substrate thickness different from 1.2 mm is reproduced, a ratio of the incident light amount of the lens portion to the total light amount of the reproducing spot is made higher than, or equal to 90%.
While the information recorded on the substrate having the thickness of 1.2 mm is reproduced by employing these objective lenses,. the light amount of the light entered into the peripheral portion is reduced or interrupted.
Furthermore, either this light amount reducing means or light interrupting means is moved in conjunction with the tracking operation of the objective lens to the information pit.
Also, the objective lens is made with either the light amount reducing means or light interrupting means in an integral form.
In the case that the information recorded on the substrate having the thickness of 1.2 mm is reproduced, a dimension of a photodetector and a dimension of an optical system are optimized as other means for reducing the light in the peripheral portion.
An optical head capable of reproducing signals from optical recording mediums has substrate thicknesses of 1.2 mm and 0.6 mm by employing these objective lenses.
Also, in the optical head, in order to optically record/reproduce the information pit through a transparent parallel plane substrate, a paraxial focus of the objective lens for focusing laser light onto an information recording film surface on the side of the light source is separated from a surface of the objective lens on the side of the light source by at least 2 mm; the optical head further comprises means for reducing a light amount of light or for interrupting the light incident to a peripheral portion of the objective lens; a galvanometer mirror is employed as an actuator for tracking the light spot to the information pit array on the optical recording medium; and a rotary shaft of the galvanometer mirror is arranged near the paraxial focus of the objective lens on the side of the laser light source.
Also, in the above-described objective lens, the paraxial focus of the objective lens on the side of the light source is separated from a surface of the objective lens on the side of the light source by at least 2 mm.
Further, while using this objective lens, in order to reproduce information recorded on a substrate having a thickness of 1.2 mm, the optical head further comprises means for reducing a light amount of light or for interrupting the light entered into a peripheral portion of the objective lens; a galvanometer mirror is employed as an actuator for tracking the light spot to the information pit array on the optical recording medium; and a rotary shaft of the galvanometer mirror is arranged near the paraxial focus of the objective lens on the side of the laser light source.
When two sets of optical disks having different substrate thickness from each other are reproduced by employing a single objective lens, it is not necessarily avoidable that aberration may occur at least in one of these optical disks. Thus, a description will now be made as to what degree a focused light spot is deteriorated in case that aberration exists, and what degree a signal quality of an optical disk is lowered with respect to no aberration. Conventionally, as the evaluation index of the focused spot, there are root mean square wave front aberration, and the Strehl intensity related to one-to-one correspondence as follows: ##EQU3##
Since these items are such an index used to compare the magnitudes of aberration under a certain single numerical aperture and a certain single wavelength, these items do not constitute such an index used to judge the degree to which collective spots having different wavelengths and different numerical apertures, containing aberration, are reduced. Accordingly, the concept of the Strehl intensity is expanded in order that this comparison can be made for definition purposes. As is well known in the field, in case of no aberration, a dimension of a light spot is directly proportional to .lambda./NA. As a consequent, assuming now that a light amount of light entered into an incident pupil of an objective lens, it is conceivable that a central intensity of the light spot is inversely proportional to an area of the collective spot. Thus, this must be directly proportional to a square value of NA/.lambda.. In consideration of influences caused by the aberration, a central intensity in case of having aberration with respect to the central intensity of no aberration corresponds to the Strehl intensity. As a consequence, in the case that there is an index for judging focusing conditions of the light spot containing certain aberration and different wavelengths and also different numerical apertures, such a value found by multiplying the Strehl intensity by the square of NA/.lambda. is acceptable. This is referred to as an "expanded Strehl intensity", which will be defined as follows: ##EQU4##
When a wavelength and a depth "d" of an information pit of a CD are virtually changed, the amplitude of a reproduction signal is also varied. Assuming now that a refractive index of a substrate is "n" and a depth of an information pit is "d", it is known that the amplitude of the reproduction signal is changed in proportion to approximately sin.sup.2 (2.pi.nd/.lambda.). As a consequence, such a value obtained by multiplying the expanded Strehl intensity by this value is referred to a "performance factor". This performance factor is expressed as follows: ##EQU5## As a consequence, reproduction signals for optical heads containing aberration, different numerical apertures, and different wavelengths can be estimated. In other words, even when there is aberration, there are probabilities that such conditions can be found, namely the same signal qualities as those of no aberration with the different wavelengths and the different numerical apertures.
In such a case that an objective lens is subdivided into a central portion and a peripheral portion, and optimum thickness of this substrates for optimally collecting this laser light are different from each other, the light in the peripheral portion becomes no aberration when the laser light is focused through a predetermined and optimized substrate thickness of this peripheral portion. However, since spherical aberration will occur in the central portion whose substrate thickness is optimized and different from the substrate thickness, the aberration over the entire lens pupil containing the central portion and the peripheral portion does not become zero.
Generally speaking, in order that the root mean square wave front aberration is minimized with respect to the Seidel's spherical aberration, i.e., EQU W=W.sub.40p.sup.4 (6),
both defocusing and a phase shift may be applied in such a manner that the expression for the Zernike's spherical aberration has the following shape: ##EQU6## In this expression characters "W.sub.40 " indicates the Seidel's aberration coefficient of the spherical aberration, and ".rho." shows a radius coordinate when a pupil radius is selected as 1. At this time, the root mean square wave front aberration is given as follows: ##EQU7##
Also, in order that root mean square wave front aberration is minimized when spherical aberration only given to the central portion is viewed as the entire pupil, in such a case that both defocusing and a phase shift are applied in such a manner that the above-described wave front aberration defined by the expression 7 becomes only in the central portion, the aberration at the peripheral portion makes 0. Assuming now that a divisional radius of a central portion and a peripheral portion is set to "R", and the W.sub.40 is newly set as a Seidel's aberration coefficient of the spherical aberration at the radius "R", this aberration function is given as follows: ##EQU8##
At this time, root mean square wave front aberration as the entire pupil is given in a similar manner as follows: ##EQU9##
In accordance with the present invention, this root mean square wave front aberration is made less than or equal to approximately 0.04.
The reason why this value is selected is given as follows: In general, the Marechal's criterion such that the root mean square wave front aberration is less than or equal to 0.07.lambda. is widely utilized as a dimension of aberration. However, in an optical head, it is required to be suppressed to this value involving various factors other than an objective lens. These factors are, for instance, defocusing; spherical aberration caused by head adjusting shifts; astigmatism of a semiconductor laser; coma aberration caused by a disk inclination; spherical aberration caused by a disk substrate thickness shift; and also spherical aberration, coma aberration, and astigmatism caused by an objective lens forming error. There are 11 items in total by including the aberration according to the present invention. Assuming now that all of these contributions are equal to each other, since it is conceivable that the entire variance caused by the errors of the various factors is equal to a summation of variance, approximately 0.021.lambda. is acceptable as the root mean square wave front aberration allowable to the errors, according to the present invention, based upon the following expression: ##EQU10## As a consequence, the aberration owned by the objective lens is defined as follows. If the spherical aberration, the coma aberration, and the astigmatism caused by the forming errors are involved with the aberration according to the present invention, then the variance becomes 4 times. In other words, as the root mean square wave front aberration, it must be less than or equal to 0.042.lambda., i.e., two times.
In such a case that information is reproduced through a substrate having a thickness of 1.2 mm, light passing through only a central portion is used. At this time, the thickness of the substrate at which the light passing through only the central portion becomes no aberration is not always equal to 1.2 mm as an optimum thickness. As previously explained, this is because the root mean square wave front aberration must be less than or equal to approximately 0.04.lambda., involving the lens forming error, when the signal is reproduced through such a substrate thickness that no aberration results at the peripheral portion by way of the light of the overall pupil combining the central portion with the peripheral portion. However, if the substrate thickness is excessively deviated from 1.2 mm, then the spherical aberration will be increased when the signal is reproduced through the substrate having the thickness of 1.2 mm by employing only the central portion.
An allowable value of spherical aberration for the light passing through this central portion may be determined based upon such a condition that a reproduction signal similar to that of the conventional CD is obtained. First of all, since the wavelength of the light used in the conventional CD is 0.78 .mu.m and the numerical aperture is 0.45, it can be understood that such spherical aberration is allowable within a range that the performance factor is obtained under this condition. The performance factor of the CD without any aberration is equal to PF=(0.45/0.78).sup.2 sin.sup.2 (.pi./3)=0.250(.mu.m.sup.-2), since the above-described wavelength, numerical aperture, and pit depth "nd" is about .lambda./6. Also, in the case of aberration existing, it is conceivable to obtain a performance factor having the same or larger value under a shorter wavelength or a larger numerical aperture. Normally, also in the CD, if it is conceivable that the Strehl intensity becomes 0.8 at minimum obtained from 10 items, i.e., 7 items of the defocusing; spherical aberration; coma; astigmatism caused by shift of head adjustment; astigmatism of the laser; coma aberration caused by the disk inclination; and the spherical aberration caused by the disk substrate thickness deviation, and other than the object lens, and 3 items caused by the spherical aberration caused by shift of forming the object lens; astigmatism; and coma aberration, the Strehl intensity is 1-(1-0.8)/10.times.3=0.94 taking account of the influences of the aberration caused by only the objective lens. In this case, the performance factor becomes 0.250.times.0.94=0.235(.mu.m.sup.-2). As a result, it can be expected that if the performance factor is greater than or equal to approximately 0.235, then the reproduction signal performance equivalent to that of the CD can be achieved.
As explained above, even when the spherical aberration is left in the central portion, it could be represented that if the DVD is reproduced with either the shorter wavelength or the larger NA than that for the CD, then the reproduction signal equivalent to that of the CD can be obtained. As a consequence, since the spot diameter of .lambda./NA must be less than or equal to the value of CD, i.e., 0.78/0.45=1.733 .mu.m, the numerical number only for the central portion must be larger or equal to 0.57.lambda. (".lambda." is indicated in unit of .mu.m), depending upon the magnitude of the aberration.
Since all of the above descriptions are related to the performance of the light on the axis, this objective lens should be designed so as to substantially satisfy the Abbe's sine condition as the entire lens in order that the performance of the light outside the axis is assured to some extent.
Normally, since an objective lens is processed by way of molding, if the objective lens contains a stepped portion, then such objective lens can be hardly processed for the sake of molding. As a consequence, also as to the objective lens according to the present invention, shapes of boundary portions between peripheral portions and central portions must be connected in a smooth manner within a range where the above-explained performance is not deteriorated.
The above description describes that the region of the objective lens is subdivided into two regions. The larger the dividing number is increased, the more the free degree of designing is increased, so that more suitable designs are available. As a consequence, the optimum substrate thickness of the central portion may be eventually, continuously variable from the lens center in a coaxial shape.
Also, in order to form a stepped portion of a boundary portion between a peripheral portion and a central portion, after a lens having no stepped portion has been formed, a thin film is loaded on this lens, so that a desirable phase shift may be applied.
As another method, for such an objective lens, all surfaces which are optimized as to the optimum substrate thickness of the above-described central portion, a parallel plate having a thickness equal to a difference between the optimum substrate thickness of the central portion and the optimum substrate thickness of the peripheral portion may also be employed, into which a hole is formed at a center portion thereof on the side of this disk.
In an optical head with employment of these objective lenses, when information recorded on the substrate having the thickness of 1.2 mm is reproduced, it cannot be avoided that aberration of light in a peripheral portion is considerably increased. As a consequence, there are some possibilities that the adverse influences of this region should be removed. At this time, it is required to employ such a means for reducing a light amount of light, or for interrupting the light entered into the peripheral portion.
This may be used to design a dimension of a photodetector and an optical system. It should be noted that since there is actually an adverse influence of diffraction light by an optical disk, strictly speaking, the peripheral light cannot be completely eliminated only by the limiting aperture employed in the detection optical system. However, this peripheral light may be reduced to some degrees.
Also, there is the following conventional problem. That is, while a rotation center of an optical disk is deviated from a rotation center when a guide groove of the optical disk is cut, if the objective lens is moved in response to this decentering for the tracking purpose, then the intensity distribution of the reflection returning light is shifted. Accordingly, an offset may be produced in a push-pull tracking signal. This problem becomes serious in such a case that the above-described means for interrupting the light in the peripheral portion is not moved in conjunction with the objective lens. If the double servo system is introduced, then this decentering does not make up a serious problem, because the course actuator substantially may trace, and the objective lens does not substantially move. This double servo system has been widely utilized in, especially, magnetooptical disk units marketed as computer external storage units. However, since low cost is required for music CDs, the double servo system is not employed in the present models, resulting in a problem.
As to this problem, the light interrupting means may be moved in conjunction with the objective lens. Alternatively, the light interrupting means may be moved in the integral form with the objective lens.
As another method, this problem may be solved not by moving the lens, but by employing such an actuator that a moveable mirror called as a "galvanometer mirror" so as to vary the inclination of the light entered into the objective lens, and thus the light spot is moved. To this end, a rotation center of the galvanometer mirror is positioned on a focal point of an objective lens on the side of the light source. When this galvanometer mirror is positioned in the above-described manner, since the light on the optical axis is necessarily entered into the optical disk unless this optical disk is inclined, there is no shift in the intensity distribution of the light again reflected from the galvanometer mirror and then returned. To this end, in order that the galvanometer mirror is arranged at a paraxial focus of the objective mirror on the side of the light source, this paraxial focus on the side of the light source must be separated from the surface of the objective lens on the side of the light source more than 2 mm on the side of the light source. This is useful not only for the above-described objective lens according to the present invention, but also for such an objective lens. That is, an overall surface of this objective lens is optimized to become a substrate thickness other than 1.2 mm, and a numerical aperture thereof is greater than that for the CD, and also a CD reproduction is carried out only in the central portion of this objective lens. This is, of course, useful for such a case that the objective lens according to the present invention is employed.